Two broad classes of opal glasses have been recognized in the art for many years. The first of these, viz., spontaneous opal glasses, comprise compositions which develop opacity as the molten batch is cooled to a glass article; the opacity being the result of the growth of crystals in the glass or from some other type of phase separation occurring within the glass. The second category, termed thermally opacifiable or reheat opal glasses, includes compositions which exhibit little or no opacity when the molten batch is cooled to a glass article but, when the glass article is subjected to temperatures in the vicinity of or slightly above the annealing point of the glass, crystal growth or some other phase separation phenomenon will occur to generate opacity. In both categories of glasses, the crystal content or other phase separation developed is customarily quite small, commonly less than 10% by volume, such that the overall physical properties of the glass, other than optical transmission, will be affected only slightly, if at all. Whereas spontaneous opal glasses are generally less expensive to produce, the crystal growth or other phase separation phenomenon can be more carefully controlled in the reheat opal glasses, thereby yielding opacities of very uniform density.
Dense white glasses have been and are currently used extensively in dinnerware applications. Most generally, those glasses have been prepared from silicate-based compositions wherein sodium fluoride or calcium fluoride crystals are precipitated. As the cost of energy has risen, there has been an increased desire to develop glasses manifesting the lowest melting viscosity possible while still maintaining dense white opacity and satisfactory chemical durability.
One feature of NaF-type opal glasses which must be examined is their susceptibility to weathering. Hence, in the presence of moisture, Na.sup.+ ions will be drawn out of the surface of the glass unless the overall glass composition is designed to tie up those ions to thereby inhibit that migration.